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Old 06-30-2019, 06:57 PM   #1
northerngmc
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Replacement Exhaust manifold plug size

Truck is a 1968, C20 with the factory 396. Left exhaust manifold cracked so I bought a replacement and installed it. The new manifold has the AIR holes and the large threaded home near the exit point. I have straight pipe plugs in the 4 AIR holes, and that works great.

Any idea what size and thread the hole is near the exhaust exit?
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Old 06-30-2019, 08:10 PM   #2
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Re: Replacement Exhaust manifold plug size

Do you have a picture? I'm not familiar with anything but the 4 A.I.R. ports and the exhaust port. Maybe it's for an Oxygen sensor?
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Old 06-30-2019, 08:12 PM   #3
northerngmc
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Truck is a 1968, C20 with the factory 396. Left exhaust manifold cracked so I bought a replacement and installed it. The new manifold has the AIR holes and the large threaded home near the exit point. I have straight pipe plugs in the 4 AIR holes, and that works great.

Any idea what size and thread the hole is near the exhaust exit?
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Looking for the plug that is pictured on the left side of the photo.
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Old 06-30-2019, 08:23 PM   #4
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Do you have a picture? I'm not familiar with anything but the 4 A.I.R. ports and the exhaust port. Maybe it's for an Oxygen sensor?
I am thinking you might correct. Looks like sensor plugs are 18mm X1.5 thread.
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Old 06-30-2019, 09:13 PM   #5
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Re: Replacement Exhaust manifold plug size

Actually for the smaller tube holes, the size is 5/16'' tubing plug. If you tighten the pipe plugs too much you can crack the manifold. Yeah, big hole is for an O2 sensor. Size as stated---should have had a plug in the box when you purchased it, just like mine did
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Old 06-30-2019, 09:55 PM   #6
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Actually for the smaller tube holes, the size is 5/16'' tubing plug. If you tighten the pipe plugs too much you can crack the manifold. Yeah, big hole is for an O2 sensor. Size as stated---should have had a plug in the box when you purchased it, just like mine did
Thanks. Wasn’t a plug in the box.

I didn’t use tapered pipe thread I used straight pipe thread. So it wouldn’t tight and has to use something to lock the threads in place.
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Old 06-30-2019, 11:29 PM   #7
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Re: Replacement Exhaust manifold plug size

There are 5/16" plugs that have the same thread as a tubing B-nut, and have the inverted flare on the end to match what is machined into the exhaust manifold. Straight pipe plugs will probably work fine, though I haven't ever done it that way. Looks like you have a universal manifold that fits a ton of years.
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Old 07-01-2019, 03:09 AM   #8
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Re: Replacement Exhaust manifold plug size

Quote:
Originally Posted by Steeveedee View Post
Looks like you have a universal manifold that fits a ton of years.
That's just one of the wonderful beautiful things about BIG BLOCK Chevrolet's...
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Old 07-03-2019, 09:37 AM   #9
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Re: Replacement Exhaust manifold plug size

Found a oxygen sensor plug yesterday and it fits great. One quiet ride again.......
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Old 07-03-2019, 09:49 AM   #10
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Re: Replacement Exhaust manifold plug size

Excellent!
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